This invention relates in general to digital waveform synthesizers and in particular to waveform synthesizers for producing a waveform having angle modulation of a selected type using integrated circuitry of minimal complexity.
In the communications industry it is often desirable to generate a signal that is selectively modulated either by frequency modulation or phase modulation, both of which are types of angle modulation, or by amplitude modulation. This is because the signal propagation conditions at any given time affect the degree to which one type of modulation produces more favorable information transfer results than another. Thence, a communication system that can switch to the most favorable type of modulation for the current propagation conditions can optimize the rate and accuracy of information transfer over a communications channel.
In addition, it is often desirable in modern communication systems to generate a signal by digital waveform synthesis. This is because digital waveform synthesizers provide excellent frequency stability, waveform control and modulation flexibility.
Digital waveform generators of the prior art typically generate a sequence of digital numbers representative of the change in phase of the signal to be produced for each pulse of a periodic clock signal, that is, the frequency of the waveform, which numbers are accumulated to produce a corresponding sequence of digital numbers representative of the phase of the waveform. The phase numbers are typically applied to a phase-to-amplitude conversion means to produce a sequence of digital numbers representative of waveform amplitude. For example, the phase-to-amplitude conversion means may be a memory which stores a sequence of numbers representative of the amplitude of a sinusoid corresponding to phase numbers ranging from 0 degrees to 2.pi. radians of phase. The output of the memory, or other phase-to-amplitude conversion means, is applied to a digital-to-analog converter, whose output is filtered, to produce an analog transmission signal.
A typical prior art system which permits modulation of a digitally-synthesized waveform employs: (1) a first register for receiving a digital number representative of the center frequency of the output waveform; (2) a second register for receiving a digital number representative of the amount of frequency modulation; (3) a first adder for adding the number in the first register to the number in the second register to produce a digital number representative of the phase increment of the signal to be produced for each clock pulse; (4) an accumulator for receiving and accumulating the phase increment numbers to produce a sequence of digital numbers representative of phase; (5) a third register for receiving a digital number representative of the amount of phase modulation; (6) a second adder for adding the number in the third register to the phase numbers to produce, for each clock pulse, a number representative of the phase of the modulated waveform to be generated; (7) a means for producing a number representative of waveform amplitude for each waveform phase number; (8) a fourth register for receiving a digital number representative of the amount of amplitude modulation; (9) a multiplier for multiplying the waveform amplitude number by the amplitude modulation number to produce a number representative of the amplitude of a modulated digital signal; (10) a digital-to-analog converter; and (11) a filter for producing an analog signal corresponding to the amplitude-modulated digital signal. Examples of such systems can be found in Crooke U.S. Pat. No. 3,633,017 entitled "Digital Waveform Generator" and McCune, Jr. U.S. Pat. No. 4,746,880 entitled "Number Controlled Modulated Oscillator". However, such systems have a significant drawback in that they require an adder after, as well as before, the phase accumulator to accomplish both types of angle modulation, that is, phase modulation and frequency modulation. This takes up more space than a single adder when such an oscillator is fabricated on a single integrated circuit.
In practice, the modulation signal in such systems is frequently generated by a microprocessor or by a digital state machine. In such cases, the modulation signal is not an analog signal but a digital signal which is sampled at the modulation sample. For example, in a multi-level symbol modulation scheme, the modulation sample rate is the symbol rate times a predetermined number of samples per symbol. A digital state machine which generates a modulation signal with a predetermined number of samples per symbol is disclosed in Genrich U.S. Pat. No. 4,873,500 entitled "Phase Accumulation Continuous Phase Modulator". In systems where the modulation is presented as a sequence of digital numbers, it is desirable to load these numbers into the modulator synchronously at a periodic rate. Any variation in the rate at which these numbers are loaded degrades the modulated waveform.
One method for permitting the selection of either frequency modulation or phase modulation in a digital waveform synthesis system while eliminating the need for an adder after the accumulator for phase modulation is to use only one adder before the accumulator, but to control how the adder is used. A system of this type is shown in Ciardi U.S. Pat. No. 4,745,566 entitled "Angle Modulated Waveform Synthesizer". However, the system in Ciardi has two significant drawbacks which prevent it from providing waveform generation in a communication system using minimal circuitry. First, Ciardi discloses a signal generation device which provides a carrier signal angle modulated by a sine wave signal, rather than by a non-periodic signal as would be the case in a communication system. Second, while Ciardi selectively permits either frequency or phase modulation using only one adder which is placed in front of the accumulator, it uses more complex circuitry than is necessary. That is, for both frequency and phase modulation it employs a delay register and an arithmetic-logic unit connected to the adder, and for frequency modulation it requires a divider as well.
Therefore, it would be advantageous to have a simpler circuit architecture which selectively provides either frequency or phase modulation using only one adder in front of an accumulator and which is adapted for use in modulating a carrier signal with a non-periodic modulation signal generated by a microprocessor rather than merely a sine wave. Furthermore, it would be advantageous to have a circuit architecture which allows each number in a sequence of phase modulation or frequency modulation numbers to be synchronously loaded into the circuit by a periodic signal at a predetermined rate.